Two-way communication system



P 1940- H. w. AUGUSTADT 2,214,804

' TWO-WAY. COMMUNICATION SYST F11! July 18, 1959 INVENTOR I FIG. .3 l-l.WAUGUSTADT ATTORNEY 4,0, being accomplished Patented Se t. 11, 1940UNITED STATE TWO-WAY com'ruulca'rron srs'rau HerbertW. AngultadtvalleyStream, N. Y., as-

signer to Bell Telephone Laboratories, Incorpoi-ated, New York, Y.,'

York

a corporation of New 1 J llian... July 1%, 1939, Serial No; 2 5,042

' This inventionrelates to two-way communicationsystems and moreparticularly to communication terminal circuits. Still morespecifically. it relates to ieedback'stabilization for a combined signaland control amplifier in voice operated systems with applications totwo-way communication;

Whilethe invention has applicability in a wide variety of communicationcircuits, it will be described more specifically in connection withatelephone conference circuit. In two-way communication terminals,especially it used in conference circuits where the connection between-a plurality of terminals is more or less permanent, it is desirablethat when no normal signal is taking place the system shall be as quietas possible, being substantially immune from room, noise and otherbackground disturbances. Even during periods of signaling, suchas speechsignaling, it is desirable that in the pauses of the usual speech thecircuits shall be brought to ahighly quiescent condition. It isdesirable also'that at no time shall the power level oi the signal riseto such values as to overload any part of the signaling equipment.Furthermore, it isnecessary, if; the terminal is a two-way circuit, thatthe high gain which is characteristic amplifiers in such circuits shallnot lead to singing.

In my invention there is present in a signal path a vacuum tubeamplifier, the output load 01 which comprisesa communication line orsome signal responsivedevice. In order to maintain the system in a quietcondition during intervals at no signal, there is inserted in thesignalpath "a loss device or attenuator which in my invention may be aseries resistance or a shunt reslstonce, or a combination of the two.When signal is initiated-in this path the loss is removed, this by acontrol circuit, the input of which is itself connected in parallel tothe main load. Also connected across the output. of the amplifier and inparallel to the main load isa circuit which limits-the power deliveredby the plifier to the main load. Thus there are two control circuits inparallel with the load,

However, the cutting out of the loss referred to above substantiallyalters the impedance into which the amplifier works, causing a drop inoutput voltage which may be of suiilcient magnitude to seriously afiectthe operation of the control circuits. A part of my invention resides.in/sta ,bilizlng the amplifier circuit in such manner that the outputvoltage is nearly independent. of the fvariations in load impedance.-This stabiliza- I my invention:

- lying a stage amplifier thus causing a drop tion is accomplishedbymeans of a negative ieed-. back circuit to be described in greaterdetail.

. In addition, it the. system is a'tw'o-way terminal having a transmitside -and a receive side, the control circuits on the one side, whensubjected 5 to signal, operate to. disable the circuits on the otherside;

The inventicnwill be more fully understood by' reference to thefollowing specification and accompanying drawing in which: l0 Fig. 1 isa simplified schematic circuit showing Fig. 2 is a more detailed circuitdiagram; and v Fig. 3 is explanatory of the principles underpart of myinvention.

Referring more particularly to Fig. 2, there is shown at At an initialamplifier, pre'ierably'of the'variable mu type. While a variety of suchtubes is available, the one which I have illus trated inthls figure isthat commonly identified a the SL7 tube. The output of A1 is impressedon an amplifier A: which is here shown as a twocomprising the amplifiersAs and A3". In the output of the latter stage there is shown atransformer T1 feeding into a line, the line including a loss device Kl.here shown as aseries resistance and a shunt resistance. The loss K1 isof such a magnitude that signal initiated in this path will be soattenuated as to be of relatively small magnitude on arrival at anotherterminal. For the satisfactory transmission of signal, therefore, it isnecessary. to reduce the effective value of this loss andI accomplishthis by shorting out the whole or a part of this loss,' in this case theseries portion of the resistance being shorted .out by a relay. Tothi's'e'nd there is connected across the output circuit of As", inparallelto'the line load a circuit comprising a potentiometer P1 whichin turn applies potential to the input circuit of the amplifier Aa-Theal- 4o ternating current output of this last amplifier is rectifiedwith any suitable form of rectifier, such as the tube A1, giving adirect current drop across resistance R1. This drop .may then be applieddirectly to the winding 1 of a relay R1 or preferably, as shown, may beimpressed on the input circuit of an amplifier. Ao-operating virtuallyas a direct current amplifier which,'inturh, gives power to the relaywinding 3. The relay upon excitation closes the contacts 5 shorting outper-- tion oi-the loss K1. .It will be appreciated that the removaloi anappreciable portion of the loss K1 materially 'af-. fects theimpedanceof the main load circuit,

in the output voltage-of the.

\ 5 the following:

amplifier A3" and this drop may be suflicient to cause a release of thecontrol circuit operating on energization of coil 3, thus giving rise toa chattering of the relay with accompanying un- 5 desirable eflects.

A second control circuit is connected across the output of the amplifierAs", this circuit containing a rectifier such as the diode All. Thevoltage drop thusset up from the output of diode I Alracross resistanceR1; is applied to the grid gs of the variable mu tube Al, the bias beingsuch as to reduce the gain when the signal volume becomes excessive. Theoperation of this volume limiting device is also subject to variationswhen 15 the load impedance changes.

To stabilize the action of the circuit as a whole I provide a negativefeedback over a portion of the circuit, such as across a part or acrossthe whole of the amplifierv As, the feedback in 20 thi circuit beingshown as a network consisting of the blocking condenser Ca andresistance R3. The effect of this stabilizer is that of making theoutput voltage of A3" practically independent of the variations in loadimpedance, thus removing 25 the dimculties referred to.

"ter understood-by analysis with reference to Fig. 3, which shows asimplified circuit-diagram of a portion of the circuit underconsideration, and in connection with which an analysis on the basis ofa numerical example will be'given. 'It is t0 be understood, however,that the invention is in no way restricted by the values used.

For thepurposes of the analysis let R be the equivalent plate circuitgenerator impedance.

Also for illustrative purposes assume the following impedance effects.

these being reasonable values for the circuits and .tubes which may beused. For convenience also one may use the following notations:

V1=voltage at X required to operate the relay. 45 Vz=voltage at X afterrelay operates when Va=voltage at X after relay operates when f=20R.Eo=equivalent plate circuit generator to pro- 5 duce voltage V when j= rE=equivalent plate circuitgenerator to produce voltage V when f=20R.

.The analysis of feedback circuits then leads to Voltage V to operaterelay is the same in both cases, hence the-percentage voltage change isThe action of this stabilizing'circuit will be bet- From this it is seenthat without the stabilizing circuit there would be a 45% change "in theoutput voltage of the amplifier. As" whereas with the stabilizingnetwork this variation is reduced to 4.8% for the particular values usedin this 5 illustrative case.

In Fig. 3 one tube only is shown whereas in Fig. 2 the feedback is overthe two amplifiers As and A3. Whether the one or the other connection ismade will be determined by the sensitivity re- 'quired for suitablestabilization. In either case consideration must be given to the properphas ing of the feedback connection. Thus in Fig. 3 the particularfeedback network should be connected. directly to thegrid of the oneamplifier whereas in Fig. 2 the-feedback over two stages should be madetothe cathode for proper pliesing. Also, while in both of these figuresa very simple form of negative feedback is shown, it

is to be understood that other more complicated 1 networks may bedesirable in certain cases.

If the terminal is to be arranged, for two-way 1 communication as shownin Fig. 2, a similar cir=- cult is provided for the receive side. Thusincoming signal is impressed on the input circuit v of the tube As.which in turn is shown as connected to a second stage A4. To the outputof this stage there is connected the transformer T2 feeding into theline through a loss device K:

similar to K1. A control circuit is also provided analogous to thecontrol circuit of the transmit side already described. Operating on therelay R2 this controls the contact 6 to short out a portion of the lossm when signal comes through the receive path. 85 l In order that thereshall be no singing around the circuits of the terminal it is desirablethat when signal is going out over the transmit side, the receive sideshall be disabled. To this, end

two additional sets of contacts 1 and 9 subject 40 to the relay winding3 are provided and when the relay is operated the contacts 1 close toconnect the control grid of A: to ground and the contacts 9 close toconnect the grid of tube Aoto ground, thus disabling botlrof thesecircuits.

Similarly, if signal is passing over the receive side then the operationof its control circuit closes. 1 i I two additional sets of contacts 8and ID 'of relay R: which respectively connect the grid of the tube .1 AA3" and the grid of the tube As to ground, thus j disabling the transmitside.

In the operation of the rectifiers. it is desirable that they'shall notrespond to undesired disturbancesv of relatively small magnitude. Thiscan be accomplished by suitable biasing ofthese rec- 1:1 tifiers. Thusin the rectifier A'l instead of con-' v V necting one terminal of theresistance R/: to

ground it is shown as connected to a point above ground on the batteryB5 of As, the'biasing being 4$ such that no current flows through therectifier" o unless, the impressed voltage is in excess of bias.

limiting voltage to be impressed on the grid as of..

amplifier A it'may be desirable that no limitin 6a m action shall occuruntil the signalpower level at the output of As" exceeds a certainvalue. To this end a biasing battery Eumay be inserted in the-circuit ofthe tube All, the polarity'being such as to preventrectification untilthe im'- 7ocuit through a hybrid coil H but it-isto be under.- v

I This or similar provision is in general-pro-i 5 vided on all therectifiers. Thus in the case of the amplifier in shunt with said loadcircuit and restood, at course, that one may leave the system as afour-wire circuit. Also, while the invention has been describedprimarily in terms of a terminal circuit, it would serve equally well asa repeater station at any intermediate point on the transmission line. 1

- The schematic drawing oi, Fig. 1 is a simplifled diagram of a two-waytelephone system which include a terminal A having circuits such asillustrated in Fig. 2, shown in single line sche-.

matic form, anda plurality of other similar terminals B, C and D whichmay be associated with 1 each other and terminal A over the two-way lineL.

While in the specification. the change in im- Dedance has been describedas that due to the insertion or removal of a loss device as a functionof time, it will be recognized that the impedance variation may comeabout in other ways. For example, the impedance of a certain element inthe'load circuit might be a function 01' frequency and my circuitarrangement would still be eil'ctive for such impedance changes.

What is claimed is:

1. In combination, an amplifier, a plurality of load circuits connectedin parallel to the output of said amplifier, atleast one of said loadcircuits having a variable impedance, and means to prevent anysubstantial change in the voltage input to one ofsaid load circuits dueto the varying impedance of another load circuit comprising means fornegatively feeding back the alternating voltage across the output ofsaid amplifier to the input thereof so as to maintain the amplifieroutput voltage substantially constant with variation in the impedance ofsaid other load circuit.

2. In a signal transmission system, a signal transmission circuit, anamplifier in said circuit, a load circuit associated with the output ofsaid amplifier, a control circuit having its input associated with theoutput of said amplifier in shunt with said load circuit and responsivetoimpressed signals to function as a control in said system, and meansfor substantially preventing variation in the input to said controlcircuit due to variations in the impedance of said load circuit,comprising means for negatively feeding back voltage from the output ofsaid amplifier to,

the input thereof so as to maintain the output voltage of said amplifiersubstantially independent of variations in the impedance of said load Icircuit.

a. In 'a signal transmission system, a ma transmission circuit, anamplifier in said circuit,

I a load circuit containing a loss device connected to the output ofsaid amplifier, a control circuit having its input connected to theoutput of said ables the amplifier and the control in the impedance ofsaid load circuit sponsive to impressed signals to remove the lossfromsaid load circuit and means for substantially preventing variationin the input'to said control circuit due to variations in the impedanceof said load circuit with removal of the loss there from comprisingmeans for negatively feeding back voltage across the output of theamplifier to the input thereof so as to maintain the output the receiveside of a twoeway communication system. and further characterized by thefact that eachcontrolcircuit when performing its func-' tion of removingthe loss on its one side also discircuit on the other side.

-' 6. In a signal transmission system, a .signal transmission circuit,-an amplifier in said circuit, a load circuit containing a loss deviceconnected to the output of said amplifier, a plurality of controlcircuits connected to the output of said amplifier in shunt to said loadcircuit and responsive to impressed signals to remove the loss from theload circuit and to limit the amplitude of the signal impressed on theinput of said amplifier,

means for substantially preventing variation in the input of saidcontrol circuits due to variations with removal of saidloss therefromcomprising means for negatively'feeding back thevoltage acres the outputof said amplifier to the input thereof so as to maintain the outputvoltage of said amplifier substantially constant with variation inimpedance of said load circuit.

7. In a telephone communication system, a plurality of two-waycommunication terminals adapted to be associated with each other forintercommunication, each terminal comprising atransmit side and areceive side, each side being characterized by the combination of claim6 and further characterized by the fact that when one of the-controlcircuits on the one side is functioning to remove loss from its side-itdisables. the IMaxie amplifier and control circuits on the other e. a

T w. AUGUB'I'AUI'.

